1. Field of the Invention
The present invention relates to a magnetoresistive element having an alloy cap layer and a magnetic random access memory (magnetoresistive random access memory) (MRAM).
2. Description of the Related Art
A magnetic random access memory (MRAM) is a memory device which uses magnetic elements having a magnetoresistance effect as cell units to store information. Magnetic random access memories have received a great deal of attention as next-generation memory devices featuring high-speed operation, large capacity, and nonvolatility. The magnetoresistance effect is a phenomenon that when a magnetic field is applied to a ferromagnetic material, the electrical resistance changes in accordance with the magnetization direction in the ferromagnetic material. A magnetic random access memory can be operated as a memory device (MRAM) by recording information by using such a magnetization direction in a ferromagnetic material and reading out the information on the basis of the magnitude of electrical resistance corresponding to the information.
In recent years, in a ferromagnetic tunnel junction including a sandwich structure having an insulating layer (tunnel barrier layer: to be referred to as a barrier layer hereinafter) inserted between two ferromagnetic layers, a magnetoresistive ratio (MR ratio) of 20% or more can be obtained by a tunnel magneto-resistance (TMR) effect. With this as a momentum, magnetic random access memories which use magnetic tunnel junction (MTJ) elements using the tunnel magneto-resistance effect have been receiving expectation and attention.
When an MTJ element is used in a magnetic random access memory, one of two ferromagnetic layers sandwiching a barrier layer is formed as a magnetization reference layer by using a magnetization pinned layer in which the magnetization direction is fixed and does not change. The other ferromagnetic layer is formed as a free layer by using a magnetization free layer in which the magnetization direction readily reverses. When a state wherein the magnetization direction in the reference layer and that in the free layer are parallel and a state wherein the magnetization directions are anti-parallel are made to correspond to binary numbers of “0” and “1”, respectively, information can be stored. When the magnetizations are parallel, the resistance of the barrier layer is low, and the tunnel current is large, as compared to the anti-parallel state. Recording information is written by reversing the magnetization direction in the free layer by an induced magnetization which is generated when a current flows to a write interconnection provided near the MTJ element. Recorded information is read out by detecting a change in resistance by the TMR effect. Hence, the free layer preferably has a high resistance change ratio (MR ratio) by the TMR effect.
When the free layer is located above the magnetization pinned layer, a cap layer is often formed between the free layer and an upper interconnection layer or an etching mask. The upper interconnection layer or etching mask sometimes also serves as the cap layer. The main role of the cap layer is to prevent any degradation in magnetization of the free layer, which would be caused by element diffusion from the upper layer in the heating process or process damage in the upper interconnection layer formation process. In addition, increasing the thermal stability by preventing element diffusion from the cap layer itself to the free layer, and preventing any decrease in MR ratio by interaction with the free layer are necessary. However, no method of satisfying both of them has been reported yet.
As described above, in the cap layer of the MTJ element used in the conventional magnetic random access memory, improvement of the thermal stability by preventing element diffusion from the cap layer to the magnetic layer and prevention of any decrease in MR ratio caused by abnormal oxidation of the free layer are not implemented simultaneously.
References of prior arts related to the present invention are Jpn. Pat. Appln. KOKAI Publication No. 2005-032780, U.S. Patent Application Publication No. 2005/0008849, and Jpn. Pat. Appln. KOKAI Publication Nos. 2002-208119, 2002-050011, 2001-331908, and 2004-172599.